1. Field of the Invention
The invention relates to the field of surgery for human beings and other animals in which elastomeric materials are utilized as adhesives and as fillers to occupy spaces produced or permitted during surgery, particulary where bone replacement prosthenses are being inserted in a human being or other animal.
2. Description of the Prior Art
Artificial bones, particularly joints, have been used for many years as replacement parts for human beings and other animals. For example, a patient with degenerative arthritis at an advanced state may not be able to use a degenerated joint, so that it is often necessary to replace a degenerated joint with an artificial joint. Some of the common joints which are surgicially replaced for these purposes are knees, shoulders, hips, fingers, toes and elbows. Each type of joint requires a different type of joint replacement member. However, all types of joint prostheses share some common characteristics. The material of the prosthesis is usually metal, although ceramics or other materials may be used. For example, titanium and various types of chromium steel alloys are commonly used for prostheses. The prostheses may be solid, or they may be fenestrated (formed with windows) to allow the regenerative bone tissue to form through the windows in the prosthesis, to provide added stability and strength to the bond between the bone and the prosthesis, and to allow the luting agent to extend through the window to provide a more secure affixation in place of the prosthesis.
A prothesis, as inserted for use, includes a narrow, elongated shank which projects into a generally similarly shaped, excavated cavity which the surgeon creates in the bone into which it is to be anchored. The area which is excavated to provide the space for the prosthesis shank is called the medullary cavity. The size of the medullary cavity is not necessarily proportionate to the size of the shank and it may be substantially larger. The reason for this is that the shanks come in limited numbers of sizes, and when the surgery is performed, all of the soft cancellous tissue in the bone proximate the shank must be excavated in the area of the shank, in order to provide the most secure bond between the bone and the prosthesis.
The luting agent acts as a filler, to fill the space between the cortical or reamed section of the bone and the shank of the prosthesis. This function is obviously critical for several reasons. First of all, the material used as a luting agent must be relatively liquid or amorphous in its initial state, so that it can easily be inserted in and fill completely the medullary cavity which has been formed. It must also, when cured, conform closely and snugly to the shape of the shank of the prothesis. The luting agent must also provide good adhesive properties to the prosthesis and to the human or other animal bone in question. The luting agent, during the polymerization reaction in which it solidifies and hardens, must not produce an excessively high exotherm, because the exotherm could do enormous heat damage to the adjacent tissue. The luting agent should desirably have isotropic elastic properties which are similar to the elastic properties of the contiguous bone, so that shock absorption and therefore stress in the area, when the prothesis is in use (as, for example, when a person with a knee prosthesis is running) is substantially uniformly distributed through the bone and through the hardened luting agent, a condition which is necessary to maintain maximum bone strength. Lastly, the luting agent should not be friable, because particles which break away from a luting agent are foreign bodies within the joint, and can cause severe abrasive tissue damage.
Many polymeric materials have been attempted for use as luting agents, some with reasonable success. However, none of the luting agents of the prior art is capable of achieving satisfactorily all of the foregoing desiderata. Each has numerous shortcomings.
Polymethylmethacrylate is the most commonly used luting agent. Its limitations are: it is exothermic in use, producing temperatures of approximately 100.degree. C. which can cause tissue trauma; it is brittle and, under the stress of use, can fragmentize and these fragments can accelerate the wear of the adjacent components of the prosthesis and cause tissue damage; it shrinks substantially during polymerization, reducing the intimate contact surface area between the luting agent and the bone and creating stresses within the polymer itself which increase the likelihood that the polymer will disintegrate, break down and fragmentize; and it may cause systemic hypotension during insertion.
In joint replacement surgery, there is a failure rate of almost one hundred percent of all joint replacements within five years after they have been inserted. This failure rate is caused by the loosening of the prosthesis as a result of the loosening of the luting agent, due to shrinkage, loss of physical properties and physical disintegration. Also, polymethylmethacrylate is not an elastomeric material, so that with the normal stresses to which joints are subject during extensive use, the polymethylmethacrylate cannot adapt, causing it to fracture and break apart.
There are a couple of other materials which have been utilized as luting agents to a limited extent and without particular success. One of these is a material called "Ostamer," which is essentially a rigid polyurethane foam, formed by the reaction of toluenediisocyanate and water, and which has been used to repair bone fractures. In its use, several substantial parallel grooves are sawed into the broken bone in the direction of the elongated axis of the bone and extending beyond the broken areas of the bone. Metal rods are inserted in these grooves and held in place by metal bands proximate the ends of these rods. The Ostamer is then inserted to fill the sawed apertures in the bone and to hold the metal rods in place, forming a rigid polyurethane foam to hold the fractured ends together. This use of Ostamer was not acceptable for several reasons, including the very high exotherm produced during formation of the rigid polyurethane foam, which caused extensive tissue damage. Also, the raw materials used in creating the Ostamer are toxic and the polymer, when formed, is not biostable. Therefore, this procedure is no longer in use.
Polyester has also been attempted for use as a luting agent, without satisfactory success, because of its high exotherm, its high shrinkage, its extreme brittleness, and its chemical instability at body temperatures.
Reference is made to U.S. Pat. No. 4,267,299, which discloses 100% solid polyurethanes and poly(urea)urethanes formed by spray application.